added significant terms aggregation

src/Aggregation/SignificantTermsAggregation.php

+<?php
+
+/*
+ * This file is part of the ONGR package.
+ *
+ * (c) NFQ Technologies UAB <info@nfq.com>
+ *
+ * For the full copyright and license information, please view the LICENSE
+ * file that was distributed with this source code.
+ */
+
+namespace ONGR\ElasticsearchDSL\Aggregation;
+
+/**
+ * Class representing TermsAggregation.
+ *
+ * @link https://www.elastic.co/guide/en/elasticsearch/reference/current/search-aggregations-bucket-significantterms-aggregation.html
+ */
+class SignificantTermsAggregation extends TermsAggregation
+{
+    /**
+     * {@inheritdoc}
+     */
+    public function getType()
+    {
+        return 'significant_terms';
+    }
+}